Fuel control systems for aircraft are generally of a mechanical type wherein fuel flow is regulated by a direct linkage between the pilot's power input lever and the output lever connected to the aircraft engine. The direct linkage results in a 1:1 displacement ratio between the input and output levers. Thus, for every degree of movement of the pilot's input lever, a corresponding and equal displacement of the output lever will be achieved through a linear linkage.
In a typical fuel control system for an aircraft, the initial portion of the stroke of the levers brings the engine from full reverse horsepower to a ground idle condition. The second portion of the stroke raises the engine horsepower from the ground idle mode to minimum flight horsepower. The remaining portion of the stroke is used to control engine horsepower in flight. In normal operating procedures a pilot uses full reverse horsepower for landing and a small range of ground idle speeds for taxiing. These maneuvers require relatively few adjustments of the input lever. In contrast, during flight, a wide range of speeds and therefore shaft horsepower are utilized. With the prior art linear linkage system, power management in flight is confined to the last third of the stroke of the input lever. Since small movements of the pilot's input lever result in significant changes in the engine horsepower, fuel and power management in flight is difficult.
Various means have been suggested for modifying the displacement ratio between the input and output levers, in order to achieve greater sensitivity in flight. More specifically, it would be desirable to provide a means which varies the displacement ratios of the levers throughout the input stroke such that during the first portion of the stroke, wherein the engine horsepower is raised to flight speed, a given displacement of the input lever results in a greater amount of displacement of the output lever. In contrast, in the latter portion of the input stroke, for controlling shaft horsepower in flight, a given displacement of the input lever results in a smaller amount of displacement of the output lever. By this arrangement, small variations in shaft horsepower in flight could be readily achieved by the pilot.
Various mechanical or electromechanical equipment may be designed to accomplish the above stated objective. For example, the throttle control can be electronically or hydraulically activated or a gear system could be utilized. However, besides the high cost of the latter expedients, a major shortcoming associated therewith is their relative unreliability as compared with a linkage mechanism. Even a relatively simple gearing system is considered to be a hundred times less reliable than a linkage system. Similarly, hydraulic and electronic systems which are inherently far more complex, are even less reliable. As can be appreciated, high reliability is of premium concern when dealing with the power control of an engine in an aircraft.
Accordingly, it is an object of the subject invention to provide a new and improved linkage system having high reliability which is operative to vary the displacement ratio characteristics between input and output levers.
It is another object of the subject invention to provide a unique linkage system arranged such that the amount of displacement of the output lever during the first portion of the stroke is greater than the displacement of the input lever.
It is a further object of the subject invention to provide a unique linkage system arranged such that during the second portion of the input stroke, used for controlling flight, the displacement of the output lever is less than the displacement of the input lever thereby increasing the sensitivity of the input lever for improved in-flight engine power management.
It is still another object of the subject invention to provide a new and improved linkage system which functions to vary the displacement ratio characteristics of the input and output levers and in addition increases the mechanical advantage of the input lever during the latter portion of the input stroke to aid in the pilot's control.